1. Field of the Invention
This invention relates to a machine comprising lightweight, high strength pistons with or without piston rings, operating in a reciprocating internal combustion engine cylinder block or liner, and more specifically to a machine using pistons and cylinder blocks or liners fabricated from carbon--carbon composite materials.
2. Description of the Related Art
Internal combustion reciprocating engines used for aerospace, military, and transportation applications must be lightweight and capable of operating at elevated temperatures and pressures. Under the current state-of-the-art, the relatively high temperatures and pressures associated with operation of a reciprocating internal combustion engine necessitates pistons made of either aluminum alloys, cast-iron, and/or steel. However, engine pistons manufactured of steel and/or aluminum alloys are heavy which adds weight to the reciprocating mass of diesel and gasoline engines. Steel and aluminum alloy pistons are also highly thermally conductive; hence, a significant heat transfer, i.e. heat loss, through the cylinder wall results. In diesel engines this "through-the-wall" heat loss reduces engine efficiency.
Cylinder blocks for reciprocating internal combustion engines in automobiles typically have been made of cast iron because of the need for high mechanical strength. Use of cast iron, however, adds weight to the engine and results in lower fuel economy. In an effort to reduce engine weight, various light-weight alloys such as aluminum alloy have been used to fabricate the cylinder block. Typically, the engine block mass is made of aluminum alloy and a thin-walled cast iron sleeve is inserted to line the cylinder bore(s). Alloys of aluminum are lighter than cast iron, however, they have a lower mechanical strength which creates undesirable vibration. In addition, aluminum alloys inherently possess lower temperature resistance and a higher coefficient of thermal expansion (CTE) than cast iron which means that differential thermal expansion between aluminum alloys and cast iron must be taken into account in design.
The inherently high coefficient of thermal expansion of aluminum alloys necessitates larger clearances between an aluminum alloy piston and a cast iron cylinder wall, to avoid piston scuffing and/or sizing which could occur as an aluminum alloy piston expands during high temperature engine operation. In order to seal the clearance, or gap, between an aluminum alloy piston and a cast iron cylinder wall, piston rings are required. Metallic and ceramic piston rings commonly are used in conjunction with steel and/or aluminum alloy pistons. Typically, ceramic rings replace metal rings when extreme operating temperatures so dictate. Ceramic rings, however, become brittle during extensive operation at extreme temperatures and are unreliable.
At operating temperatures above 300 degrees Celsius (C), the mechanical strength of aluminum alloy pistons decreases dramatically. The uppermost compression ring cannot be located too close to the crown because the reduced mechanical strength will result in deformation of the piston above the top ring due to forces exerted by ring friction. The need for positioning the top ring further from the crown increases the crevice volume between the piston and cylinder wall which, by necessity, must exist to accommodate thermal expansion of the piston. A further disadvantage of larger gaps between aluminum alloy pistons and the cylinder wall includes "piston rocking" in the cylinder bore which increases engine noise and necessitates additional piston mass as longer skirts are needed. Large amounts of lubricants are also required to control the wear rates of the piston and cylinder wall.